Proximal FAD histidine residue influences interflavin electron transfer in cytochrome P450 reductase and methionine synthase reductase

- Reciprocal active site substitutions in CPR (H322A) and MSR (A312H) were created.
- Interflavin electron transfer was inhibited in CPR H322A and accelerated in MSR A312H.
- Placement of His in active site weakens coenzyme-binding affinity.
- His322 likely positions conserved residues of catalytic triad for rapid deprotonation of FADH2.